Subtropics to Visit Alaska

Given that the weather in Salt Lake continues to be dominated by the passage of just some occasional "cirrus monotonous", I'm looking elsewhere for some interesting weather and the winner is Alaska. 

Things are setting up for a major surge of moisture into south central Alaska.  The 0600 UTC GFS shows the setup with a pronounced upper-level short-wave trough approaching the state from the southwest at 1200 UTC 28 October (4 AM Alaska time Monday morning) and low-level fetch extending from the tropics over the Gulf of Alaska and into the state's interior.

What is most impressive about forecasts of this event is the penetration of moisture into the high latitudes.  Over interior Alaska and the Yukon, for example, the forecast precipitable water (a measure of the total water vapor in the atmosphere) reaches 0.5 to 0.75 inches at 4 AM Monday morning (solid contours below).  Such values aren't all that unusual over the Gulf of Alaska, but are unusual for this time of year in the Alaskan and Yukon interior.  In fact, in some areas they have never been observed over the past 30 years during the 3 week period centered on Monday (color fill below).

Source: NWS

Precipitation over mountainous regions is not only influenced by the amount of moisture, but also the transport, which is related to the wind speed. As shown below, there is an elongated filament of high moisture transport (solid lines) known as an atmospheric river extending from the subtropics into the Alaskan and Yukon interior.  The values forecast over a wide region of the interior haven't been observed over the past 30 years during the 3 week period centered on Monday (color fill below - see red area).

So, this has the potential to be an unusual weather event for late October over southcentral Alaska, especially the interior.  There is some uncertainty with regards to the forecast, however, so, as noted by the National Weather Service in their discussion this morning:

"THE FULL EXTENT OF THE IMPACTS ARE UNKNOWN AT THIS TIME BUT SHOULD BECOME MORE CLEAR IN THE COMING DAYS."